Interpretive Summary: The declining Ogallala Aquifer of the U.S. Southern High Plains limits crop irrigation for more dependency on rain. We adapted a 3-year rotation for growing wheat, and corn plus a fallow by adding deficit irrigation of 1 or 2 in. every 10 days. We also compared disk (DT), stubble-mulch (SM) or no (NT) –tillage effects on water savings and corn water use on a Pantex silty clay loam at the USDA-ARS Conservation and Production Research Laboratory, Bushland, TX. Wheat straw cover with NT increased soil water stored during fallow up to 2 inches over bare DT. Less growing season evaporation with NT also increased corn water use and grain yield. Our corn water use was from 54% to 85% of crop need. Corn grain yield was between failure and half of full irrigation levels. We conclude that NT had less water loss from the soil than DT for more corn water use. Greater corn water use with NT residue also increased yield, but did not offset grain yield reductions under deficit irrigation.

Technical Abstract: Storing precipitation as soil water during crop-rotation fallow periods may offset decreasing irrigation well capacity that is caused by the declining accessible groundwater of the Ogallala Aquifer in the Southern High Plains. A three year dryland rotation that produces crops of wheat (Triticum aestivum L.) followed by corn (Zea mays L.) with intervening 10-month fallow periods was adapted for use with deficit irrigation at capacities of 2.5 and 5.0 mm d**1 on a Pantex silty clay loam (fine, mixed, superactive, thermic Torrertic Paleustoll) at Bushland, TX (35 degrees 11' N, 102 degrees 6' W). Study objectives were to quantify disk (DT), stubble-mulch (SM), or no (NT) -tillage effects on storage, as soil water, of precipitation during the fallow after wheat and the water use and grain yield of the subsequent deficit irrigated corn crop. The 4-yr mean soil water content after fallow ranged from about 58 to 79% of the storage capacity, but increased from 14 to 50 mm for SM and NT practices compared with DT. Cumulative growing season water use ranged from about 54 to 64% of the estimated crop water use, ETc, for the 2.5 mm d**1 irrigation capacity and from 76 to 85% for 5.0 mm d**1 irrigation with corresponding corn grain yields that ranged from a crop failure to about 7.4 Mg ha**1. We conclude that residue retaining conservation tillage increased soil water storage during fallow and partitioned early growing season evaporation from the soil to increase corn transpiration and yield. Increased soil water and greater transpiration using conservation tillage practices did not offset the corn ET reduction imposed by either deficit irrigation capacity and resulted in unprofitable crop yield reductions.